A catastrophic optical damage (COD) of a semiconductor laser device during high power operation results from the band gap energy of the active layer being smaller at facets than at a middle part of the active layer due to the presence of surface states in the vicinity of the facets. More specifically, the temperature of the laser locally increases at the facets due to absorption of laser light at the facets during the high power operation. This excess increase in the temperature reduces the band GaP energy at the facets, so that the absorption of the laser light is encouraged and the temperature at the facets further increases. At last, the laser facet is melted, resulting in an irreversible destruction. The light output power at which the facet destruction, i.e., COD, occurs is called the COD threshold optical power. This COD threshold optical power limits the maximum output power of a semiconductor laser comprising AlGaAs or AlGaInP system materials.
In order to prevent COD, the band gap energy of portions of the active layer where surface states will be produced should be increased compared to the band gap energy of the other portions of the active layer. For example, an extended abstract No. 29a-SA-4 of the Japan Society of Applied Physics (Spring Meeting, 1990) discloses an AlGaInP system semiconductor laser including a window structure in which the band gap energy of an active layer is higher in regions in the vicinity of facets than in a central region, whereby the COD threshold optical power is significantly increased to increase the output power of the laser.
When GaInP or AlGaInP is grown under prescribed growth conditions, a natural superlattice structure in which atoms are periodically arranged is formed. When an impurity, such as Zn, is selectively doped into a region of the natural (spontaneous) superlattice structure to disorder the superlattice, the disordered region has a band gap energy larger than that of the other regions. In the above-described publication, an active layer comprising GaInP or AlGaInP is grown under the growth conditions that produce the natural superlattice structure and, thereafter, an impurity is doped into a region of the active layer in the vicinity of the laser facet to form the window structure.
FIG. 10 is a sectional view of an AlGaInP system semiconductor laser having a window structure fabricated by the disordering of the natural superlattice structure, taken along the resonator length direction of the laser. In the figure, reference numeral 101 designates an n type GaAs substrate. An n type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P lower cladding layer 102 having a thickness of about 1.5 .mu.m and a carrier concentration of 5.times.10.sup.17 cm.sup.-3 is disposed on the substrate 101. An undoped Ga.sub.0.5 In.sub.0.5 P quantum well (QW) active layer 103 having a natural superlattice structure is disposed on the lower cladding layer 102. The active layer 103 is about 70 nm thick. A p type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P upper cladding layer 104 having a thickness of about 1.5 .mu.m and a carrier concentration of 1.times.10.sup.17 cm.sup.-3 is disposed on the active layer 103. A p type Ga.sub.0.5 In.sub.0.5 P band discontinuity reduction (hereinafter referred to as BDR) layer 115 having a thickness of about 0.1 .mu.m and a carrier concentration of 1.times.10.sup.18 cm.sup.-3 is disposed on the upper cladding layer 104. A p type GaAs contact layer 105 having a thickness of about 3 .mu.m and a carrier concentration of 1.times.10.sup.19 cm.sup.-3 is disposed on the BDR layer 115. An n side electrode 106 is disposed on the rear surface of the substrate 101, and a p side electrode 107 is disposed on the contact layer 105. Zn diffused regions 108 are disposed in the vicinity of the laser facets. The active layer 103 includes portions 109 where the natural superlattice structure is disordered. Reference numeral 120 designates emitted laser light.
FIGS. 12(a)-12(d) are sectional views illustrating process steps in a method of fabricating the window structure AlGaInP laser shown in FIG. 10. In these figures, the same reference numerals as in FIG. 10 designate the same or corresponding parts.
Initially, there are successively grown on the n type GaAs substrate 101 the n type AlGaInP lower cladding layer 102, the GaInP QW active layer 103, the p type AlGaInP upper cladding layer 104, the p type GaInP BDR layer 115, and the p type GaAs contact layer 105, producing the laminated structure shown in FIG. 12(a). Preferably, these layers are grown by MOCVD (Metal Organic Chemical Vapor Deposition). The growth condition of the active layer 103 is controlled so that the active layer has a natural superlattice structure.
Thereafter, as shown in FIG. 12(b), an SiO.sub.2 pattern 110 having an opening 110a in which the laminated structure is cleaved along the line of alternating long and two short dashes. The width w of the opening 110a is about 20 .mu.m considering the precision of the cleaving process.
In the step of FIG. 12(c), using the SiO.sub.2 pattern 110 as a mask, Zn atoms are selectively diffused into the laminated structure by a vapor phase or solid phase diffusion technique, forming a Zn diffused region 108. The superlattice structure of the active layer 103 is disordered at a portion 109 due to the Zn diffusion. The diffusion rate of Zn atoms in GaAs is different from that in AlGaInP. That is, in this structure, the diffusion rate of Zn atoms is higher in the AlGaInP QW active layer 103 and the upper and lower AlGaInP cladding layers 104 and 102 than in the GaAs substrate 101 and the GaAs contact layer 105.
After removal of the SiO.sub.2 pattern 110, the n side electrode 106 and the p side electrode 107 are formed on the rear surface of the substrate 101 and on the contact layer 105, respectively. Subsequent to the formation of the electrodes 106 and 107, the resonator facet 150 of the semiconductor laser is formed by cleaving (FIG. 12(d)), completing the semiconductor laser shown in FIG. 10.
A description is given of the operation. When a forward bias voltage is applied across the n side electrode 106 and the p side electrode 107, electrons and holes are injected into the active layer and recombine to produce light. The light thus generated travels along the active layer between the opposed resonator facets 150. When the amplification rate exceeds a threshold, i.e., when the current flowing in the forward biased laser exceeds a threshold current, laser oscillation occurs. Since the natural superlattice in the regions 109 of the active layer 103 in the vicinity of the resonator facets 150 is disordered by the Zn diffusion, the band gap energy of the regions 109 is larger than that of other regions. Therefore, in this prior art laser, the COD threshold optical power is significantly increased to increase the output power of the laser.
FIG. 11 is a sectional view illustrating another AlGaInP system semiconductor laser with a window structure according to the prior art, taken along the resonator length direction of the laser. Also in this laser, the window structure is formed by disordering the natural superlattice structure. In the figure, the same reference numerals as in FIG. 10 designate the same or corresponding parts. Reference numeral 118 designates Zn diffused regions produced at the laser facets.
FIGS. 13(a)-13(d) are sectional views illustrating process steps in a method of fabricating the semiconductor laser shown in FIG. 11. In the figures, the same reference numerals as in FIG. 11 designate the same or corresponding parts.
Initially, as illustrated in FIG. 13(a), an n type AlGaInP lower cladding layer 102, a GaInP QW active layer 103, a p type AlGaInP upper cladding layer 104, a p type GaInP BDR layer 115, and a p type GaAs contact layer 105 are successively grown on an n type GaAs substrate 101 preferably by MOCVD. The growth condition of the active layer 103 is controlled so that the crystal structure of the active layer becomes a natural superlattice structure.
Thereafter, as illustrated in FIG. 13(b), resonator facets 150 are formed by cleaving the laminated structure at a position shown by the line of alternating long and two short dashes.
Thereafter, Zn atoms are diffused from the resonator facet 150 of the laser structure by a vapor phase or solid phase diffusion technique, forming a Zn diffused region 118 as shown in FIG. 13(c). The natural superlattice structure of the active layer is disordered by the Zn diffusion at a region 109 in the vicinity of the laser facet 150. The depth of the impurity diffusion is controlled so that the length of the disordered region 109, i.e., the window region, is about 4.about.5 .mu.m.
To complete the semiconductor laser, an n side electrode 106 and a p side electrode 107 are formed on the rear surface of the substrate 101 and on the contact layer 105, respectively (FIG. 13(d)).
A description is given of the operation. When a forward bias voltage is applied across the n side electrode 106 and the p side electrode 107, electrons and holes are injected into the active layer 103 and recombine to produce light. The light thus generated travels along the active layer between the opposed resonator facets 150. When the amplification rate exceeds a threshold, i.e., when the current flowing in the forward biased laser exceeds a threshold current, laser oscillation occurs. Since the natural superlattice structure of the active layer 103 is disordered due to the Zn diffusion at the regions 109 in the vicinity of the resonator facets 150, the band gap energy of the active layer 103 is larger in the regions 109 than in the other region. Therefore, also in this prior art laser, the COD threshold optical power is significantly increased to increase the output power of the laser.
In the prior art window structure visible light semiconductor lasers shown in FIGS. 10 and 11, the window structure for preventing COD is realized by the impurity diffused region 109 of the active layer 103 in which the GaInP crystal structure is disordered, utilizing the fact that the band gap energy of the GaInP active layer increases when the GaInP crystal structure is disordered. That is, the band gap energy of the undoped GaInP layer grown by MOCVD is 1.86 eV whereas the band gap energy of the Zn-diffused, i.e., disordered, p type GaInP layer is 1.93 eV, that is, 70 meV larger than 1.86 eV.
In the prior art window structure semiconductor lasers fabricated utilizing the disordered superlattice, Zn atoms are diffused into the QW active layer 103 to disorder the quantum well structure in a region in the vicinity of the resonator facet, and the Zn diffused region 108 (118) extends from the contact layer to the substrate. Since the Zn diffused region 108 (118) has a relatively high carrier concentration and a relatively low resistance compared to the other regions, current injected from the electrode easily flows through this region 108 (118). Therefore, a portion of the current injected from the electrode becomes a leakage current 130 flowing through the Zn diffused region 108 as shown in FIGS. 14(a) and 14(b). The leakage current 130 does not contribute to the light output of the laser. In the prior art window structure semiconductor laser, the threshold current of the laser or the current required for a desired output power unfavorably increases due to the leakage current, resulting in an increase in power consumption.
In the prior art semiconductor laser shown in FIG. 10, when the Zn atoms are diffused from the wafer surface to disorder the superlattice structure, if the diffusion front can be stopped in the AlGaInP lower cladding layer 102, the leakage current is reduced. However, the controllability of the diffusion depth is usually low. Especially, it is very difficult to stop the diffusion front in the AlGaInP lower cladding layer 102 because the diffusion rate of Zn atoms in AlGaInP is very high. Therefore, in many cases, the diffusion front does not stop in the AlGaInP lower cladding layer 102 and reaches the substrate 101. Furthermore, since the position of the diffusion front varies in a wafer, laser devices with uniform characteristics are not produced.
FIG. 9 is a graph illustrating the relationship between the PL (photoluminescent light) peak energies of Ga.sub.0.5 In.sub.0.5 P and (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P layers and angles of GaAs substrates from the (100) surface toward the [011] direction, disclosed in IEEE Journal of Quantum Electronics, Vol. 27, No. 6, June 1991, pp. 1483-1489.
As shown in FIG. 9, when Ga.sub.0.5 In.sub.0.5 P layers are grown on a just (100) GaAs surface (off-angle=0.degree.) and on a surface forming a prescribed angle with the (100) surface (hereinafter referred to as an off-angled surface) by MOCVD at a growth temperature of 680.degree. C. and a V/III ratio (the ratio of group V materials to group III materials) of 550, the Ga.sub.0.5 In.sub.0.5 P layer grown on the off-angled surface has a band gap energy larger than the band gap energy of the Ga.sub.0.5 In.sub.0.5 P layer grown on the just (100) surface. When the off-angle exceeds 7.degree., the difference in the band gap energies between these layers exceeds 0.074 eV. The same result as described above is obtained with respect to (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P.
The above-described shifting in the band gap energy is attributed to the crystal structure of the grown layer. For example, when GaInP is grown under the above-described conditions, GaInP grown on the just (100) surface has a crystal structure schematically shown in FIG. 15 in which Ga atoms 320, P atoms 321, and In atoms 322 are regularly ordered. FIG. 15 is a projection view of a [111] superlattice on a (110) surface. The [110] direction relative to the (001) surface is equal to the [011] direction relative to the (100) surface. Hereinafter, this state of the crystal structure is called an ordered state. On the other hand, GaInP grown on the off-angled surface has a crystal structure schematically shown in FIG. 16 in which Ga atoms 320 and In atoms 322 are not periodically arranged, i.e., these atoms are disordered, increasing the band gap energy of the GaInP. The reason why the shifting of the band gap energy increases with an increase in the off-angle of the substrate is that the degree of the disordering increases with the increase in the off-angle. The relationship between an GaInP crystal structure and the shifting of the band gap energy is described in more detail in Applied Physics Letters, Vol. 59, No. 9 (1989), pp. 1360.about.1367.
FIGS. 17(a) and 17(b) illustrate a prior art window structure semiconductor laser utilizing the above-described phenomenon, disclosed in Japanese Published Patent Application No. Hei. 3-185782. FIG. 17(a) shows a cross section perpendicular to the resonator length direction of the laser, and FIG. 17(b) shows a cross section taken along the resonator length direction, i.e., along a line 17b--17b of FIG. 17(a).
In FIGS. 17(a) and 17(b), reference numeral 201 designates an n type GaAs substrate having a resonator length of 350 .mu.m and a chip width of 300 .mu.m. The substrate 201 has a (100) oriented surface in a central region intermediate the resonator facets (region A in FIG. 17(b)) and a surface inclined by 5.degree. from the (100) surface toward the [011] direction (hereinafter referred to as 5.degree. off (100) surface) in a region in the vicinity of each resonator facet (region B in FIG. 17(b)). The 5.degree. off (100) surface is formed by a conventional dry etching technique, such as RIE (Reactive Ion Etching) or RIBE (Reactive Ion Beam Etching). The length of the region B is about 20 .mu.m.
An n type Ga.sub.0.5 In.sub.0.5 P buffer layer 202 having a thickness of 0.3 .mu.m is disposed on the entire surface of the substrate 201. An n type (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer 203 having a thickness of 1 .mu.m is disposed on the buffer layer 202. An undoped Ga.sub.0.5 In.sub.0.5 P active layer 204 having a thickness of 0.08 .mu.m is disposed on the n type cladding layer 203. These layers are grown by MOCVD.
A p type (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer 205 is disposed on the active layer 204. The p type cladding layer 205 has a stripe-shaped ridge 205a extending in the resonator length direction. The p type cladding layer 205 is 1.0 .mu.m thick at the ridge 205a and 0.2 .mu.m thick at portions other than the ridge 205a. The width of the top surface of the ridge 205a is 5 .mu.m.
A p type Ga.sub.0.5 In.sub.0.5 P cap layer 206 having a thickness of 0.1 .mu.m is disposed on the top surface of the ridge 205a. An n type GaAs current blocking layer 207 is disposed on the p type cladding layer 205, contacting the opposite sides of the ridge 205a. A p type GaAs contact layer 208 is disposed on the cap layer 206 and on the current blocking layer 207.
A p side electrode 209 and an n side electrode 210, each comprising a Cr film, an Sn film, and an Au film successively deposited in this order, are disposed on the contact layer 208 and on the rear surface of the substrate 201, respectively.
In this prior art laser device, the substrate 201 has the just (100) oriented surface in the central region A intermediate the resonator facets and the 5.degree. off (100) oriented surface in the regions B in the vicinity of the resonator facets, and the respective AlGaInP layers are successively grown on the substrate 201.
As shown in FIG. 9, with an increase in the off-angle of the substrate, the band gap energies of the Ga.sub.0.5 In.sub.0.5 P layer and the (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P layer increase. The band gap energy of these layers on the 5.degree. off (100) surface is about 60 meV larger than that on the just (100) surface.
Therefore, in the prior art laser device, the energy band gaps of the respective AlGaInP system semiconductor layers are about 60 meV larger in the B regions than in the A region. Consequently, light absorption in the regions B near the resonator facets is suppressed, whereby the COD is reduced.
FIG. 18 is a graph illustrating light output power vs. current characteristics of this prior art laser device. As shown in FIG. 18, a good linearity without kinks is maintained until the light output power exceeds 30 mW. FIG. 19 is a graph illustrating a result of a life test of the prior art laser device. In the life test, the laser device is driven at a constant output power of 20 mW and a temperature of 40.degree. C. In FIG. 19, the dashed line shows a result of a life test of a laser device that is fabricated by growing a plurality of semiconductor layers as shown in FIG. 17(b) on a flat substrate and forming Zn diffused regions in the vicinity of the resonator facets to increase the band gap energy at the resonator facets.
It is found from FIG. 19 that a significant increase in the life time is achieved in the prior art device. The reason is as follows. In the laser device having the window structure formed by Zn diffusion, a lot of crystal defects caused by excess impurity doping are present in the vicinity of the laser facet, so that unwanted heat is generated in this region due to light absorption, adversely affecting the performance of the laser device. On the other hand, in the prior art laser device shown in FIGS. 17(a)-17(b), since the window structure at the laser facet is produced only by growing the semiconductor layers on the off-angled surface of the substrate by MOCVD, the crystal defects are significantly reduced, increasing life time and reliability.
As described above, in the prior art laser device shown in FIGS. 17(a) and 17(b), the window structure is produced utilizing the fact that the superlattice structure of the active layer 204 is disordered and the band gap energy is increased when it is grown on the off-angled surface. However, since the portions of the active layer 204 in the window structure, i.e., in the regions B, are grown on the off-angled surface in the resonator length direction, if the off-angle is large, light generated in the active layer in the central region A unfavorably leaks from the active layer when the light passes through the boundary between the region A and the region B. In this case, since the upper and lower cladding layers 205 and 203 in the window structure have energy band gaps larger than the energy band gap of the active layer 204, the function of the window structure is achieved by the cladding layers. However, this structure is not a window structure utilizing a disordered region of an active layer having an increased energy band gap as a window.
Further, the fabrication of the laser structure shown in FIGS. 17(a) and 17(b) includes selectively dry-etching the (100) oriented substrate to form the off-angled regions at the opposite ends of the substrate in the resonator length direction. However, it is impossible to perform the dry etching while maintaining a precise angle of the inclination. In addition, since the dry-etched surface is damaged, a crystal layer grown thereon has a degraded and uneven surface and dislocations. Consequently, it is impossible or very difficult to fabricate this structure.